The process of forming plexifilamentary film-fibril strands and forming the same into non-woven sheet material has been disclosed and extensively discussed in U.S. Pat. No. 3,081,519 to Blades et al., U.S. Pat. No. 3,227,794 to R. D. Anderson et al., U.S. Pat. No. 3,169,899 to Steuber, U.S. Pat. No. 3,851,023 to Brethauer et al. and U.S. Pat. No. 3,387,326 to Hollberg et al., all of which are incorporated by reference herein. This process and various improvements thereof have been practiced for a number of years by E. I. du Pont de Nemours and Company (DuPont) in the manufacture of Tyvek.RTM. spunbonded olefin.
Part of the foregoing manufacturing process includes a step of applying an electrostatic charge to a flattened and partially spread open plexifilamentary film-fibril strand after it is spun at a spin pack and before it is laid down on a conveyor belt. Electrostatic charges are thereby applied to the individual fibrils which cause the fibrils to repel one another, thus maintaining the separation of the fibrils in a spread apart form. The flattened strand (or probably more accurately described as a plexifilamentary film-fibril web once the strand has been flattened) is then suited to being laid down, along with other webs from adjacent spin packs onto a conveyor to form a sheet. Without the electrostatic charge, the web tends to draw together before it can be laid down causing numerous defects and very poor quality sheet products. The conveyor may also be provided with an electrostatic charge opposite to the charge on the strand thereby improving the attraction force to the conveyor and improving pinning on the conveyor. The process of applying a charge to the webs has worked quite satisfactorily in the current arrangements, although the equipment for applying the charges continue to require improvements in a number of areas.
In spite of the success and satisfaction with the overall flash spinning process and system, the process includes the use of perchlorofluorocarbon (CFC) solvents which are currently believed to cause ozone depletion and the use of which will soon be legislatively foreclosed. Accordingly, alterative solvents having suitable performance characteristics in the flash-spinning process are being aggressively sought. DuPont has expended considerable resources developing alternative solvents and has focused on several that may eventually be used commercially. As might be expected, the different solvents require some modifications in the manufacturing process or present problems that did not exist using the CFC solvents.
Hydrocarbon solvents are currently considered the most attractive alternatives to the potentially ozone depleting solvents presently in use. However, the resulting hydrocarbon atmosphere, into which the strands are spun, causes a lower charge current efficiency for the electrostatic charge applying equipment. In other words, in the process of manufacturing flash spun polyolefins, the use of promising hydrocarbon solvents reduces the effective electrostatic charge applied to the web passing through the electrostatic field for a given current as compared to the same process using a conventional CFC solvent. As a result, the webs would not be as fully opened up and the resulting non-woven sheet is less uniform than a sheet formed of more fully charged webs. Sheet uniformity is an important issue for product quality and has a substantial effect on the value of the product.
In actuality, an adequate charge can be applied to the webs by increasing the power delivered to the electrostatic charge applying equipment. However, there is a limit to the energy that can be put into the system prior to the corona field breaking down and electric arcs forming between the needles and the target plate. Also, the increased energy level causes rapid deterioration of the elastomeric coating on conventional target plates substantially decreasing pack life. The substantial expense of such short term pack life will cause unacceptable costs for the manufacture of Tyvek.RTM. material. Target plate fouling and deterioration are predicted to substantially reduce the duration for which the spin pack may be operational in a spin cell leading to substantial production cost increases.
Even if deterioration of the target plates may be resolved (such as using a metal target plate, see U.S. Pat. No. 3,578,739 to George), target plates do become fouled with a coating of polymer residue during the flash spinning process and the increased energy input increases the rate of fouling. The residue coating reduces the charging efficiency and the charging current is increased to maintain the desired charge on the web, further exacerbating the problem of fouling. When the target plate is sufficiently fouled, the system becomes unable to apply a charge to the web regardless of the charging current applied to the system. As noted above, when the electrostatic charging system for a spin pack fails, the spin pack must be shut down and replaced else it will likely create many defects in the web. Fortunately, replacement of spin packs may be accomplished during continued production of sheet material by adjusting adjacent spin packs. However, if an adjacent spin pack becomes inoperative during the replacement process, production of the sheet material is likely to be shutdown. Production shutdowns seriously effect profitability, so the average pack life of a spin pack seriously effects the economics of production.
Accordingly, it is a primary object of the present invention to provide a method and system for applying an electrostatic charge to a web in a flash spinning production operation which avoids the drawbacks as described above.
It is a more particular object of the present invention to provide a method and system for applying an electrostatic charge to a web in a flash spinning production operations which has a greater resistance to fouling as compared to current methods and systems.